HD radio technology upgrades broadcast radio from analog to digital. Audio content is digitally compressed and broadcast by a transmitter designed specifically for broadcasting digital radio signals and corresponding analog radio signals carrying the same audio content as a respective digital radio signal over the airwaves on the same broadcast frequency. Content of a digital radio signal can also include textual information. HD radio, receivers are configured for allowing the analog radio signal and the digital radio signal to be separately processed for output of corresponding content (e.g., the audio content). When the HD radio receiver is tuned to an HD station, the audio content is output using the digital radio signal unless, as described below, the need arises to output the audio content using the analog radio signal. Ibiquity Digital Corporation is one example of a source for commercial implementations of HD radio technology.
Typical HD radio receivers have a threshold for the quality of the digital radio signal they are receiving. Break-up of the digital radio signal causes dead spots in the output of the audio content corresponding to the digital radio signal. However, instead of dropping the digital radio signal that is being broadcast by a radio station and letting output of the audio content cease, the HD radio receiver makes a decision to fall back to a corresponding analog radio signal that is being broadcast by the radio station in combination with the digital radio signal.
With regard to radio signals in the broadcast frequency range of between about 540 KHz and about 1700 KHz, which is generally considered to be the amplitude modulation (AM) radio signal band, the HD radio receiver makes the decision to fall back from signal processing (i.e., audio content delivery) of a digital radio signal to signal processing of a corresponding analog radio signal based on HD signal quality such as determined by a digital signal processor of the HD radio receiver. For daytime operation of the HD radio receiver, the HD signal sensitivity threshold value is selected for maximum sensitivity, which maximizes digital radio signal reception.
At night-time, however, utilizing an HD signal sensitivity threshold value that causes the HD radio receiver to quickly switch from the analog radio signal to the digital radio signal can be problematic. This is because, after the sunset (i.e., at night-time), the strength of AM band radio signals is highly variable due to conditions such as variability in signal sky-wave propagation. Signal sky-wave propagation refers to non-line-of-sight signal transmission resulting from radio signals that reflect (e.g., skip) off of an ionosphere portion of the atmosphere and return to Earth many hundreds of miles away. However, variations occur in such signal sky-wave propagation as a function of the sun's heating of the ionosphere during the daytime and associated cooling of the ionosphere after sunset. Accordingly, AM band radio signal fluctuations resulting from variations in signal sky-wave propagation can lead to undesired transitions between signal processing of corresponding digital and analog radio signals due to degradation of HD radio signals in combination with variability in night-time AM band radio signal strength characteristic.
Prior art HD radio receivers have utilized a common HD signal sensitivity threshold value for both daytime and night-time radio operation. However, as a result of AM band radio signal strength variation, such prior art HD radio receivers exhibit the undesirable tendency of transitioning between corresponding digital and analog radio signals (i.e., analog/digital mode transitions) too often at night-time when tuned to stations in the AM radio signal band. These signal processing transitions between the corresponding digital and analog radio signals are apparent to a listener of a prior art HD radio receiver as a choppy audio content delivery (i.e., poor radio signal processing). Therefore, a solution for limiting excessive signal processing transitions between corresponding digital and analog radio signals at night-time would overcome shortcomings associated with prior art HD radio receivers, thus making such solution advantageous, desirable and useful.